This invention relates generally to continuous wave (CW) radar range measuring systems and more particularly to systems of such type wherein a transmitted CW signal is modulated in accordance with a known code and target radar returns are correlated with differently time delayed replica of the code to determine target range.
As is known in the art, CW radars have been used to accurately, and unambiguously measure the velocity of a target by detecting a Doppler frequency shift in radar returns from the target. In order to enable such CW radar system to determine the range to the target, the transmitted carrier signal frequency, or phase is modulated in some known fashion to determine target range. This technique is described on pages 86 through 111 of a book entitled "Introduction to Radar Systems" by Merrill I. Skolnik, published 1962 by McGraw-Hill Book Company, Inc.
One typical application for such CW radar range measuring system is in missile fuzing systems. In one such fuzing system, the phase of the transmitted CW signal is changed by 180 degrees, or not changed, selectively in accordance with a binary, pseudo-random noise code. Thus, a code of for example, N bits, is stored in a recirculating shift register memory. The bits are read out at a predetermined rate, f.sub.s, and fed to a bi-phase modulator along with the CW signal. The phase of the CW signal changes 180 degrees in response to a logic 1 bit and remains unchanged in response to a logic 0 bit, for example. The coded CW signal is transmitted through a transmit antenna. Radar returns from a target are received by a receive antenna. The received CW returns will have the same code as the transmitted code; however, the received code pattern will be time delayed with respect to the transmitted code an amount related to the target's range. Target radar returns are correlated with differently time delayed replica of the code to determine target range. Such technique is described on pages 10.19-10.25 in "Radar Handbook-Second Edition", edited by Merrill I. Skolnik, published by McGraw-Hill Publishing Company, 1990. Further, the frequency of the returns will be shifted by the targets' velocity (i.e., Doppler frequency of the target).
One system used to determine the target's range separates the received signals into a plurality of channels. The signal in each channel is fed to a correlator where the signal is multiplied with the code (albeit that the code fed to each channel has a different predetermined time delay relative to the transmitted code) and integrated. Thus, each channel corresponds to a range channel. In the absence of noise and/or clutter, the one of the channels having the greatest received power indicates the range to the target. (That is, the received code will "correlate" with the transmitted code in the range channel corresponding to the target's range.) Further, the frequency component in the frequency spectrum for such range channel having the greatest power indicates the target Doppler velocity (i.e., Doppler frequency, f.sub.D).
While such system may provide adequate target range measurement accuracy in some application, amplitude and phase mismatches among the channels may adversely effect such system's performance in other applications.
As is also known in the art, with such system, the bits of the code is transmitted at a bit rate, f.sub.s, and therefore the target range is resolved to a range resolution cell of 1/f.sub.s. While such range resolution may be adequate in some applications, in other applications greater range cell resolutions or accuracies are required.